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Orangutans and Chimpanzees: They’re Just Like Us!

In 2014 the Journal of Personality and Social Psychology published an article by Alexander Weiss from the University of Edinburgh and Scottish Primate Research Group and James E. King from the University of Arizona entitled Great Origins of Personality Maturation and Sex Differences: A Study of Orangutans and Chimpanzees. Weiss and King were motivated to study primates’ personality development in order to understand how and why personality develops throughout life and why some personality traits differ between males and females.

In completing their study Weiss and King found that male chimpanzees displayed higher levels of neuroticism scores than their female counter parts. They also had higher levels of overall activity and dominance as they got older than female chimpanzees, female orangutans, and male orangutans. To reach this conclusion Weiss and King had two hypotheses that they tested. One hypothesis they “offered to explain age-related changed in personality is Five-factor Theory (McCrae & Costa, 2003). It posits that personality development, like personality itself, had biological and genetic origins and is ultimately the product of evolution (McCrae & Costa, 2003). Five-factor theory thus predicts that developmental trends will differ only by small amounts across cultures.” Another hypothesis offered to explain age-related changes in personality (Roberts, Wood, & Smith, 2005) by Weiss and King is “Social-Investment Theory… it posits that personality development arises from individuals investing in age-related social roles.” An example of an age-related social role is getting a job soon after graduating college or becoming a parent once a certain age is reached. Social Investment Theory predicts that trends in development will be different across cultures in accordance with differing social structures. Evidence that Weiss and King give to support this hypothesis includes data that highlights a variety of social roles, like parenting or obtaining a job, that increases in Agreeableness and Conscientiousness as well as decreases in Neuroticism and Extraversion. “A meta-analysis of cross-sectional studies found that evidence for associations and found that, for some social roes, the associations were stronger where investment in the role was greater (Lodi-Smith & Roberts, 2007).” With humans it seems that those who are devoted to which ever social role they fulfill, they have higher levels of Agreeableness and Conscientiousness.

The main question that Weiss and King attempt to answer with their study is if the similarity between personality development in chimpanzees and humans is a consequence of how social both species are. If sociality of a species determines personality development than these similarities between chimpanzees and humans may reflect social and life events, like competing for a promotion at work, or for a status (de Waal, 2000; Goodall, 1986; Pusey & Schroepfer-Walker, 2013). A second question of gender rose from the study following the primary question. Weiss and King noticed that “male chimpanzees show a different pattern of personality development than female chimpanzees do (King et al., 2008).” To observe if and how gender had an effect on personality development Weiss and King selected male and female chimpanzees and orangutans to study their behavior under certain conditions.

To complete the study on personality maturation and sex differences in orangutans and chimpanzees, Weiss and King collected a sample of 70 orangutan males and 104 orangutan females. They were collected from 32 zoological parks located in the United States, two from Canada, one from Canada, and one from Singapore. These orangutans ranged in age from 1.8 to 51.2 years. (M=21.6, SD=12.0) They also collected a sample of 77 male chimpanzees and 125 female chimpanzees from 17 zoos in the U.S. and one from Australia. Their ages ranged from .8 to 55.2 years old. (M=16.5, SD= 12.2) The monkeys were gathered from a variety of places so that Weiss and King could observe differing levels of sociability due to cultural differences.

Weiss and King examined age and sex differences in the chimpanzee and orangutan domains Extraversion, Dominance, Neuroticism, and Agreeableness. They also examined activity and gregarious facets of Extraversion and the orangutan Intelligence domain.

All of the raters were zoo personnel or volunteers who participated in unrelated projects that involve observing chimpanzee’s behavior. Volunteers (90) rated the chimpanzees and the zoo personnel (107) rated the orangutans. To rate the chimpanzees and orangutans the raters were given questionnaires which consisted of personality descriptive adjectives like “clumsy”, “autistic”, “anxious”, “vulnerable”, “cool”, “curious”, “conventional”, “thoughtless”, “distractible”, “quitting”, “individualistic”, “innovative” and, “unperceptive”. These adjectives were all in addition to the Big Five Taxonomy. To do the analysis Weiss and King did a T-test with chimpanzee domains identified by the principal axis factoring of 43 items on 100 of the subjects in the sample. Orangutan domains were defined as 48 items on 152 of the subjects. To make the results comparable with those from human studies the t-scores were based on the means and standard deviations of human subjects who were about 18 years of age. When orangutans and chimpanzees were compared in a single model the t-test used was based on the mean and standard deviation that was derived from a combined sample of 72 female chimpanzees, 84 female orangutans, 37 male chimpanzees, and 47 male orangutans. Other analysis were generated using within-species T-scores.

Through the analysis of these T-tests they found a significant interaction that indicated declining levels of Agreeableness in orangutans but increased levels in chimpanzees, similarly to humans. They may have obtained a result like this because of how Agreeableness was rated according to the species. Results showed that Extraversion and Neuroticism declined across age groups in both species which is similar with the human comparison group. This is an interesting finding because other studies have shown that levels of Neuroticism tend to increase as a human ages. Weiss and King also found that male chimpanzees, unlike orangutans showed higher levels of Neuroticism than females. They also maintained high levels of activity and dominance as they aged than any of the other groups. The Personality-age correlations were comparable across the species and showed many similarities to the findings reported in human studies.

The findings obtained by Weiss and King in this study support their first hypothesis—the Five-Factor Theory, and suggests the role of gene-culture coevolution in shaping personality development, and suggests that sex differences evolve separately in different species.